Demand for broadband communications access is increasing. Such access is difficult to provide in some cases. For example sparsely populated rural and other underserved areas of the world lack wired infrastructure to support wire-line broadband access. The Institute of Electrical and Electronics Engineers (IEEE) Wireless Regional Area Network (WRAN) working Group proposes a standard specification (designated 802.22) for wireless networks to meet the growing demand for wireless broadband access. The IEEE 802.22 WRAN specification describes a WRAN system configured to operate within radio frequency (RF) broadcast bands typically reserved for licensed users. One example of a licensed user in an RF broadcast band is a television broadcast station.
Channel switching is an important capability for WRAN. WRAN transceiver nodes switch operating channels to avoid interfering with licensed incumbent services in broadcast bands. WRAN nodes are capable of switching from a first channel, e.g. a channel on which a node has established a communication link, to a second channel when incumbent use is detected.
Another reason for WRAN channel switching is to maintain quality of service (QoS) on WRAN communication links. Link quality can degrade due to factors such as weather, electrical interference, damaged equipment and other factors. When link quality degrades it is sometimes desirable for a WRAN system change to a different channel to maintain link quality. Channel switching supports an option to establish a new communication link on a second, different channel if the first channel degrades.
Another reason for channel switching is to employ a spread spectrum communication technique known as frequency hopping (FH). Frequency hopping is another way a WRAN can avoid interfering with incumbents. Frequency hopping WRAN systems distribute communication in the time domain over a plurality of different frequencies. Each of the plurality of frequencies is used for only a small amount of time.
Incumbents are assigned relatively narrow frequency bands. Incumbents typically have rights to transmit at a power high enough to override a WRAN communication. Therefore, any interference caused by a WRAN on a given channel which affects the incumbent is transient. Any interference from a WRAN is likely to be overridden by the incumbent. At the same time, an incumbent overrides only one of the frequencies used by a frequency hopping WRAN station. Therefore, only one part of a WRAN transmission is disturbed by an incumbent arriving on a licensed channel.
One channel switching challenge for WRAN is avoiding channel collisions with other WRAN when switching channels. If more than one WRAN station selects the same second channel for switching at the same time a collision between WRAN stations can occur. Therefore, apparatus and methods for controlling channel switching to avoid channel collisions in WRAN systems are needed.